The invention relates to a pneumatic transport device for an electrophotographic printing or copying machine, having a device for feeding back excess toner to the developer station.
In electrographic printing or copying machines, which operate in accordance with the principle of electrophotography, ionography or magnetography, a latent image, which is inked in with toner in a developer station, is produced on an intermediate substrate with the aid of a drawing-generating device. The toner image is then transfer-printed in a transfer printing station onto the recording substrate and is fixed in a fixing station. Before the application of a new latent drawing image on the intermediate substrate, it is necessary to clean the intermediate substrate of adhering residual toner. This can be carried out with the aid of a brush and vacuum in a brush cleaning station. It can also be necessary in the developer station to suck up accumulating residual toner dust and to remove it from the developer station.
This accumulated toner is normally collected in a catchbox located in the machine and thrown away from time to time. Reuse of this accumulated toner was previously not envisaged since, in machines with high requirements on printing quality, only fresh toner can be processed.
Although it is already known in the case of electrophotographic printing devices to suck up the accumulation or excess toner from the cleaning station and from the developer station, to separate it in a cyclone separator from the suction air and to feed it once more continuously via a transport device to the developer station. It has emerged, however, that, in the case of high requirements on the printing quality, such a simple feeding back is not possible. The accumulated toner contains dirt particles, used toner particles and other particles disturbing the printing process, which have a negative effect on the printing quality.
U.S. Pat. No. 3,439,630 discloses a pneumatic transport device for an electrophotographic printing machine for transporting toner and/or dirt particles out of units of the machine. The device has two vacuum-producing devices in the form of air pumps and two particle separators in the form of cyclone filters, via which the excess toner in the region of the transfer printing station and the toner located in the developer space of the developer station is sucked up and is separated into dirt particles and reusable toner. The dirt particles are intercepted in a filter box and the reusable toner is supplied to the developer space, for example each time the printing device is switched on.
Similar devices are also described in JP-A-61 100 783 and JP-A-59 143 180.
A further problem in the recycling of toner consists in the fact that, for developing the charge images in the developer station, the toner must be charged up triboelectrically, specifically in a defined manner. If accumulated toner charged up in an undefined manner is re-supplied to the developer station, this can lead to disturbances in the charge behavior of the toner in the developer station, which in turn has a negative influence on the printing quality.